Medical ultrasound imaging systems typically use a one-dimensional phased array to form an image of a two-dimensional slice through a patient's body. This approach has limitations. First, the two-dimensional slice is always perpendicular to the face of the transducer, thereby limiting the choice of views. Second, anatomy such as the left ventricle is inherently three-dimensional. To obtain an accurate volume measurement of the left ventricle, three-dimensional data must be acquired. A goal in three-dimensional cardiac imaging is to obtain a measure of the volume of the left ventricle in both end systole and end diastole, so that ejection fraction and cardiac output can be estimated.
In current solutions, such as Hewlett Packard's Sonos 5500, a two-dimensional slice of data is acquired per cardiac phase each heartbeat. For a volume of 120 two-dimensional slices, 120 heartbeats are required to gather the volume data. Cardiac, respiratory, patient whole body and sonographer motion occur during this long acquisition. Cardiac motion may be frozen by using an ECG trigger. Respiratory motion may be reduced through the use of either multiple breathholds or respiratory gating. However, since the heart does not return to the same position from breath to breath or from breathhold to breathhold, discontinuities are introduced into the acquired data. Patient and sonographer motion also cause discontinuities and overall geometric distortion.
A system capable of acquiring real time three-dimensional data by electronically steering in two dimensions is described by T. Ota et al. in "Accuracy of Left Ventricular Stroke Volume Measurement Using Real-Time, Three Dimensional Echocardiography Flow Probe in Vivo", 70th Scientific Session American Heart Association Meeting, Nov. 11, 1997. This system uses 512 active transducer elements. Signals from the transducer elements are passed through a cable having 512 coaxial conductors into a system with appropriate electronics. The image quality of the system is limited due to the small number of transducer elements used.
Apparatus for obtaining a three-dimensional reconstruction of anatomic structures through the acquisition of two-dimensional ultrasound images is disclosed in U.S. Pat. No. 5,159,931 issued Nov. 3, 1992 to Pini. One or more two-dimensional images are acquired during alternate heartbeats of a patient. When more than one two-dimensional image is acquired during a single heartbeat, the images represent a constant two-dimensional slice of the patient's heart at different phases of the cardiac cycle. During alternate heartbeats when images are not being acquired, the transducer is rotated to a new position. Rotation of the transducer may be performed mechanically using a stepping motor or electronically using a matrix or multiple arrays of transducer elements. Acquisition of a three-dimensional image requires 120 heartbeats.
All of the known prior art three-dimensional cardiac imaging techniques have had low resolution and/or long acquisition times. In the case of long acquisition times, the images typically exhibit discontinuities due to cardiac, respiratory, patient and/or sonographer movement. Accordingly, there is a need for improved methods and apparatus for three-dimensional cardiac ultrasound imaging.